1. Technical Field
This invention is related to the general field of soil remediation and to the more specific fields of soil remediation using chemical oxidizing agents.
Contaminated soil is a widespread problem resulting from years of unregulated industrialization. Because contaminated soil poses substantial health hazards, various remediation technologies have been developed to clean-up contaminated sites. Removal of the contaminated soil for disposal or treatment off-site is a generally costly procedure, particularly where the contaminated site must then be restored with clean fill. In-situ treatment is generally preferred where the conditions permit its use.
2. Related Prior Art
Chemical oxidation is known to be an effective means to treat a wide range of organic contaminants. Strong oxidants, such as ozone, hydrogen peroxide and permanganate have been suggested to treat soil and ground water contaminants via several delivery methods, including surface application, seepage, percolation, wells and push tools. Permanganate in particular is a very effective oxidizing agent for organic contaminants such as, but not limited to: benzene, ethyl benzene, toluene, xylene, methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane (TCA), carbon tetrachloride, chloroform, chlorobenzenes, ethylene dibromide, tertiary butyl ether, tetrachloroethylene (PCE), trichloroethene (TCE), dichloroethene (DCE) and vinyl chloride.
As shown in U.S. Pat. No. 6,019,548, however, there is a presumption against using large quantities of permanganate, particularly for in-situ soil remediation, because the permanganate oxidation reaction precipitates manganese dioxide, which is a solid compound at ambient temperature. Although manganese dioxide is a generally harmless and naturally-occurring mineral, the common supposition is that the total oxidant demand of the natural organic material in most contaminated soil would require the application of such a large volume of permanganate in order to oxidize the organic contaminants to an acceptably safe concentration that the resulting concentration of manganese dioxide in the soil would be unacceptable. Thus, U.S. Pat. No. 6,019,548 teaches the use of another water-soluble oxidizing agent before or with the permanganate agent to satisfy the soil""s natural oxidant demand, leaving the organic contaminants to be preferentially oxidized by the permanganate.
When using injection methods that require a permanganate oxidizing agent to migrate or percolate though the soil, such as pressure injection through wells or pipes, or attempting to seep permanganate into the soil by spreading it on the surface, the production of excess manganese dioxide may also inhibit the process. The formation of manganese dioxide by oxidation of the soil""s natural organic materials may, as U.S. Pat. No. 6,019,548 describes, form its own migration barrier by progressively reducing the soil permeability as it advances.
This migration barrier problem can be overcome if the permanganate is applied to the soil using a soil mixing device, preferably a chain trenching tool, as described in the applicant""s prior U.S. Pat. No. 5,631,160, entitled xe2x80x9cMethod for In-Situ Soil Remediationxe2x80x9d. The trenching tool chums and lifts soil to the surface and drops it back into the trench. This mechanical agitation very efficiently comminutes and aerates the soil, while injection nozzles near the chain can be used to introduce and mix the permanganate into the soil. The uniform distribution of the permanganate caused by this repeated churning of the soil brings the permanganate into close proximity to disbursed organic contaminants without having to depend upon migration through the soil.
Even with mechanical soil mixing, however, the amount of permanganate needed to treat heavily contaminated soil to attain the very low concentrations of organic contaminant that are acceptable safe target levels would still be great. Many of the highly contaminated sites have both a high concentration of organic contaminant and a high natural hydrocarbon composition. The amount of permanganate needed to oxidize to an acceptable safe level of contaminant would make the process very costly and would result in soil with an undesirable level of manganese dioxide.
These problems can be alleviated by a sequential remediation process in which hot air injection is used for thermal stripping of bulk hydrocarbon from the soil before the permanganate or other strong chemical oxidizing agent is introduced. Thermal stripping with hot air is an effective remediation technique for high concentrations of many volatile contaminants and will simultaneously strip volatile natural hydrocarbons from the soil. Thermal stripping yields diminishing returns, however, as the contaminant concentration is reduced to trace levels that are normally considered acceptable levels. The sequential process uses hot air thermal stripping until a remediation effectiveness plateau is reached, then an effective amount of permanganate or other chemical oxidizing agent is introduced and mixed into the soil. Since the bulk of both natural organics and organic contaminant have been stripped off by the hot air treatment, the amount of oxidizing agent needed to further reduce the contaminant concentration to below the target level is reasonably low.
The preferred method of injecting hot air into the soil for the initial thermal stripping is through injection nozzles adjacent a trenching tool as described in U.S. Pat. No. 5,631,160. This method and apparatus can be used for the initial thermal stripping, and for the follow-up treatment with a chemical oxidizer such as permanganate. When used in conjunction with an oxidizing agent such as permanganate, the permanganate can be injected through the nozzles, and/or hot air can be injected after the permanganate introduction to raise the temperature of the soil and accelerate the oxidation rate. In both stages, vapors from the soil can be collected through a vacuum hood disposed over the soil-mixing device as described in the above patent to contain hazardous material.
The preferred method also contemplates the possibility of pre-heating the contaminated soil in cold weather before the permanganate is introduced, particularly in clay soils with high water content. The initial thermal stripping stage does not usually require preheating, since the hot air injection produces a sufficient localized heating. The follow-up stage using permanganate, however, may be aided by pre-heating the soil, particularly when the soil is allowed to settle and cool after thermal stripping. Pre-heating thaws frozen soil, dries the soil to reduce the binding effect of moisture within the soil, and increases soil temperature to accelerate the oxidation rate. A ground heater system that uses dry, radiant hydronic heat, such as provided by Ground Heaters, Inc., may be used for pre-heating of the soil. This type of heater system includes insulation blankets and a poly vapor barrier that prevents heat loss to the atmosphere. This heater system conducts a greater percentage of the generated heat into the ground than conventional hot air heating systems, thus reducing fuel cost and time. These methods were confirmed by experimental field tests.